1. Field of the Invention
The present invention relates to a conductive paste, and to a ceramic multilayer substrate including the conductive paste.
2. Description of the Related Art
A ceramic multilayer substrate is used as a circuit board that forms, for example, an integrated circuit, a high frequency electronic circuit or a hybrid circuit. The ceramic multilayer substrate generally includes a ceramic sinter and conductive sinters formed at least at the interior and on the surface of the ceramic sinter.
The ceramic multilayer substrate is formed by preparing a plurality of ceramic green sheets; printing a conductive paste containing a metal powder having a low resistance, such as copper, on a predetermined sheet as a main component; stacking these sheets to prepare a ceramic layered product; and sintering the ceramic layered product. During the sintering, the starting temperatures and the ending temperatures of the shrinkage of each material depend on the characteristics of the materials in the sheets and the conductive paste. For example, two materials have a different sintering temperature range in which the materials shrink are sintered at the same time. As the temperature is increased during the sintering, the shrinkage of the conductive paste begins and is finished, and then the shrinkage of the sheet begins. In this case, a compressive stress due to the solidified conductive sinters affects the shrinkage of the sheets, that is, the compressive stress prevents the shrinkage behavior of the sheet from shrinking uniformly in the thickness direction and in the plane direction. Unfortunately, the sheets are warped and deformed. Furthermore, the conductive sinters are also warped and deformed due to the warp and the deformation of the sheets.
Regarding the problem caused by the difference between the sheets and the conductive paste of the temperature range in which the shrinkages are caused, Japanese Unexamined Patent Application Publication No. 11-353939 discloses the use of a conductive paste having a shrinkage starting temperature higher than the shrinkage ending temperature of the sheets. In other words, after the shrinkage of the sheets is finished, the sintering temperature is increased to gradually shrink the sheets and the conductive paste.
In this case, however, the solidified sheets after shrinking alter the shrinkage of the conductive sinters. Consequently, the conductive sinters can be peeled from the sheets. Furthermore, the ceramic after shrinking is further sintered in order to finish the shrinkage of the conductive paste by further increasing the temperature. This process changes the crystal structure of the sintered ceramic and the larger thermal energy generates glass that is brittle and fragile. This is another typical problem concerning the ceramic.